1. Field of the Invention
The present invention relates to a process in which a heavy hydrocarbon oil (hereinafter referred to as "heavy oil") is thermally cracked by using a fluidized bed of "heat carrier" particles to obtain mainly a light hydrocarbon oil which is liquid at normal temperature (hereinafter referred to as "light oil"). More particularly, the present invention relates to an improvement of the thermal cracking process comprising (1) a thermal cracking step of contacting a heavy oil with a fine powder of a porous material fluidized by a steam-containing gas to effect the thermal cracking of the heavy oil and (2) a regenerating step of fluidizing the fine powder removed from the thermal cracking step with a molecular oxygen-containing gas or a steam-containing gas and burning or gasifying coke adhering to the powder to effect removal of coke, the powder being circulated between the two steps.
2. Description of the Prior Art
Some of the present inventors clarified that, in the thermal cracking of a heavy oil by using a fluidized bed of heat carrier particles, if particles having a substantially spherical shape and a weight average particle size of 0.04 to 0.12 mm and containing particles having a size of or smaller than 0.044 mm in an amount of 5 to 20% by weight are used as particles to be fluidized, the efficiency of the thermal cracking can be increased while maintaining of a good fluidized state (see Japanese Patent Application Laid-Open Specification No. 10587/81). The present inventors have designated this process as "fluid thermal cracking process" or simply as "FTC PROCESS".
Furthermore, we found that in the above-mentioned thermal cracking process, if particles having a pore volume of 0.1 to 1.5 m.sup.3 /g, a specific surface area of 50 to 1500 m.sup.2 /g and a weight average particle size of 0.025 to 0.25 mm, which are thermally stabilized, are used as the fine powder, the thermal cracking efficiency can be further increased and that by absorption of the heavy oil in the liquid state in fine pores of the porous material, there can be attained an excellent effect of promoting the thermal cracking reaction and controlling formation of a high-carbon adhering substance (hereinafter referred to as "coke") (Japanese Patent Application Laid-Open Specification No. 18783/82). The present inventors have designated this effect as "capacitance effect".
Furthermore, in connection with the thermal cracking process comprising the step of thermally cracking a heavy oil which is combined with a gasifying step of contacting the fine powder of a porous material taken out from the thermal cracking step with an oxygen-containing gas to gasify and remove the coke adhering to the fine powder (this step is called "regenerating step" in the present invention), the fine powder being circulated between the two steps, the present inventors have proposed an effective embodiment in which a vertical reactor having at least three defined compartments in it is used at the thermal cracking step (Japanese Patent Application Laid-Open Specification No. 180590/83) and another effective embodiment in which the regeneration step is divided into a gasifying zone and a burning zone and gases generated in the two zones are separately taken out (Japanese Patent Application Laid-Open Specification No. 115387/84).
The feed heavy oil contains relatively large amounts of heavy metals such as nickel, vanadium and iron in addition to CCR (Conradson carbon residue) and a sulfur compound. In the conventional catalytic cracking using catalyst particles, and in the case where the feed oil contains large amounts of heavy metals, these heavy metals are accumulated on the catalyst and have adverse influences on the cracking reaction, as is well-known in the art. More specifically, since nickel or vanadium has a dehydrogenating catalytic activity, it excessively advances the cracking reaction of the starting oil to increase the amount of hydrogen and the amount of coke, resulting in reduction of the oil yield and quality of the cracked oil.
Adverse influences of these contaminating heavy metals cannot be avoided even in the thermal cracking using a fine powder of a porous material having no substantial catalytic activity although the degree may be lower.
As means for overcoming this disadvantage, there has been proposed a method in which a catalyst for use in the catalytic cracking of a feed oil containing heavy metals is covered with an antimony component or other transition metal component to render the heavy metals on the catalyst passivated (see, for example, Japanese Patent Application Laid-Open Specification NO. 104588/78). Furthermore, there has been proposed a method in which a catalyst withdrawn from the regenerating step is contacted with a hydrogen-containing reducing gas under conditions appropriate for eliminating influences of contaminating heavy metals and the catalyst is circulated to the cracking step (see Japanese Patent Application Laid-Open Specification No. 123289/83).
However, in the case where the cracking of a heavy oil having a high CCR content and high heavy metal contents is intended as in the present invention, the method for rendering accumulated heavy metals passive by addition of an antimony component or other heavy metal component is defective in that a large amount of the component for rendering the heavy metal passive should be used so as to control bad influences of contaminating heavy metals, and the treatment of the regenerated catalyst with a reducing gas is defective in that an additional step is necessary.
As means for thermal cracking of a heavy oil, there have been proposed a method and apparatus in which the heavy oil is contacted with a plurality of vertically piled zones having a fluidized bed of a granular carrier material at a high temperature in a gaseous atmosphere in which the hydrogen partial pressure of 2.5 to 14.1 Kg/cm.sup.2, preferably 5.3 to 10.5 Kg/cm.sup.2, and the total pressure is 10.5 to 56.2 Kg/cm.sup.2 -G, preferably 17.6 to 45.7 Kg/cm.sup.2 -G, or the total pressure is 14.1 to 56.2 Kg/cm.sup.2 or higher (see Japanese Patent Publication No. 2172/59). This method may have some problems in that gases generated at the cracking step and the regenerating step cannot be separately taken out.